1. Field of the Invention
The present invention relates to a two-port isolator, and more particularly, to a two-port isolator preferably for use in microwave frequency bands, and to communication device.
2. Description of the Related Art
Generally, an isolator allows a signal to pass only in one transmission direction and stops a signal in the other transmission direction, and is used in the transmission circuits of mobile communication equipment, such as automobile telephones and portable telephones.
A two-port isolator (e.g., an isolator having two central electrodes) has been commonly used for such isolators. This two-port isolator includes a permanent magnet, a ferrite, a first central electrode and a second central electrode disposed on a main surface of the ferrite, two matching capacitors, and a resistor mounted in a metal case including a lower metal case and an upper metal case, which are joined together. Normally, the first central electrode and the second central electrode have the same shape and the two matching capacitors have the same capacitance.
The insertion loss and isolation characteristics of such a two-port isolator used in mobile communication equipment are set according to the communication system used. Accordingly, when the insertion loss and isolation characteristics of a typical two-port isolator are compared with the requirements of a communication system, even if the isolation characteristics satisfy the requirements, the insertion loss characteristics may not fully meet the requirements. On the contrary, even if the insertion loss characteristics satisfy the requirements, there are cases where the isolation characteristics do not fully meet the requirements.
On the other hand, in mobile communication equipment, there is a strong demand for reducing the insertion loss in order to suppress the power dissipation in the transmission circuit portion and increase continuous talk time, even if the isolation characteristics are deteriorated. However, the required insertion and isolation characteristics of two-port isolators have not previous been achieved.